1. Technical Field of the Invention
This relates to an improved method and system for obtaining optical samples of good quality in optical communications.
2. Description of Related Art
Optoelectronics technology and its applications are expanding with the result that integrated optics technology can be used with considerable advantages in communications. Optical modulators, switches, multiplexers are commonly employed by fabricating them both on single substrates of both dielectrics and semiconductors. For measuring the waveforms of optical pulses used in high bit rate optical communications, it is common practice and desirable to use optical sampling with high sensitivity and high time resolution. Optical sampling systems often use a probe signal and optical mixing with a user signal to achieve what is known as sum frequency generation (SFG) which is very useful for obtaining eye diagrams for sampled user signals. SFG methods of optical sampling might result in undesirably high background noise. Higher pump powers for optical sampling systems are desirable, but result in greater need to eliminate background noise, because background noise increases as the square of the probe intensity.
It is to be noted that sum frequency generation (SFG) generally uses a sampling process in a nonlinear crystal such as a periodically poled lithium niobate (PPLN) crystal. The use of a PPLN crystal in optical sampling systems is taught for example in the publication xe2x80x9cHighly Sensitive and Time-Resolving Optical Sampling System Using thin PPLN Crystalxe2x80x9d by S. Nogiwa, et al., which is incorporated herein by reference. PPLN crystals as opposed to other nonlinear crystals, e.g., KTP (potassium titanyl phosphate), have a large sum frequency generation efficiency under quasi-phase matching conditions. By a judicious selection of the PPLN crystal thickness, reduction of the time resolution of the system and increase of the wavelength band width can be achieved.
When optical mixing is used as the sampling mechanism, the resulting xe2x80x9cbackground noisexe2x80x9d is highly undesirable. Presently, most optical sampling schemes other than those based on series connected photo-conductive switches (PCS""s) have an associated background noise problem. This is especially true of 1xc2x755 nm signal sampling instruments which are based on 1xc2x755 nm probe sources.
Optical sampling modules are commercially available with a variety of features and applications with wavelength capabilities of 1550-1650 nm and some with 1100-1650 nm. Other wavelength capabilities for commercially available optical sampling modules are also known. Also, because of the availability of optical amplifiers in the 1550 nm wavelength band, there are several technologies available for obtaining short optical sampling pulses near 1550 nm. Examples of such include gain-switched semiconductor lasers and Erbium-Doped Fiber Ring Lasers.
High-speed sampling of optical signals facilitates reliable oscilloscope measurements of sampled signals. It has been found that a narrow sampling aperture enables achieving higher bandwidths than would be achievable with regular electrical sampling techniques. Commercial short optical sampling pulse sources near the 1550 nm range may indeed be obtained for this purpose. However, in most instances the input signals to be sampled also fall in the same 1550 nm wavelength band, which makes it difficult to distinguish the input signals from the probe or sampling pulse signals, compounding the background noise problem and, complicating the measurement system design.
Thus, there is a need for sampling optical input signals of the 1550 nm wavelength range using commercially available probe/pump sources which also have the 1550 nm range. By simply using SFG and mixing the incoming signals with the user input signals, the background noise is not automatically reduced to acceptable levels. There is therefore a specific need while using commercially available probes of 1550 nm wavelength range with input signals, also of approximately 1550 nm range, to reduce background noise so as to make the output measurable and to have a clean optical sample.
This invention is directed to optical sampling from signals, using intermediate second harmonic generation, preferably using nonlinear conversion techniques. Described hereinafter are a method and apparatus which enable easy and efficient sampling of input signals which are in the 1560 nm range (i.e., 1.56 um), the probe signals also being in the 1550 to 1560 nm range. Resorting to SFG techniques, because of the close proximity between the wavelength ranges of the signal to be measured and the probe signal, it is difficult to design an optical bandpass filter. The above problem is addressed by the invention by using a frequency-doubled probe in the first stage. The invention uses intermediate second harmonic generation (SHG) of the probe pump to 0.775 micrometer (775 nm) and a fundamental wavelength rejection filter as essential components to the scheme. The 775 nm is mixed in the second stage with a user input signal of 1550 nm wavelength range to generate green light approximating a third harmonic which can be filtered to eliminate most everything other than the near third harmonic wavelength of 515 nm range. A sum frequency generation (SFG) sampling crystal is used for this purpose. The filtered near third harmonic is sensed by using a photomultiplier tube, and optionally processed using an analog-to-digital converter, and an electrical sampler with a microprocessor for possible display on a cathode ray oscilloscope.
The invention in its broad form resides in an optical sampling method by nonlinear conversion for sampling optical input signals in optical communication, comprising:
using a probe pulse source of predetermined wavelength range and frequency-doubling signals from said pulse source to obtain an intermediate output containing a second harmonic probe pulse signal, said frequency doubling being accomplished preferably using a first nonlinear crystal;
filtering said intermediate output to filter out background corresponding to said predetermined wavelength, leaving the second harmonic probe pulse signal after filtering;
mixing the filtered second harmonic probe pulse signal with a user input optical signal and obtaining a near third harmonic signal; and
processing the near third harmonic signal in a desired manner to obtain samples of the user input optical signal.
In a preferred embodiment, the step of mixing comprises using a dichroic beam splitter, and the step of obtaining the near third harmonic signal comprises using SFG in a nonlinear crystal. Preferably, the step of frequency-doubling the wavelength comprises using a nonlinear periodically poled lithium niobate (PPLN) crystal. The nonlinear crystals may comprise PPLN or other crystals of predetermined thickness and length.
Preferably, the probe pulse source comprises a passive mode-locked fiber ring laser (or, alternatively, an active mode-locked fiber ring laser), and the step of processing comprises using an analog-to-digital converter (ADC) to obtain high-speed processed sampled signals, the method further including microprocessor control and analysis. At least one advantage of the present scheme is that if a pedestal (low DC light level with a higher power spike) is present on the 1550 nm probe signal, it is considerably reduced after the second harmonic generation. This improves the dynamic range of the measurement system.
Optionally, the invention includes the step of achieving at least 50% conversion efficiency in obtaining the near third harmonic signal, and includes the step of narrowing pulse width of pulse source signals, giving greater system bandwidth.
The step of filtering may comprise using cascaded filters, and said user input optical signal may be in the 1560 nanometer wavelength range.
The invention also resides in an optical sampling system to obtain a sample from an optical input signal, comprising:
a probe pulse signal source of a predetermined wavelength range;
a frequency doubler for frequency-doubling the probe pulse signal source to obtain an intermediate output containing a second harmonic probe pulse signal;
a filter to filter said output to delete background corresponding to said predetermined wavelength, leaving a filtered second harmonic probe pulse signal;
a mixer for mixing the filtered second harmonic probe signal with a user input optical signal to obtain a near third harmonic signal; and
a processor for processing the near third harmonic signal in a desired manner to obtain samples of the user input optical signal.
In a preferred embodiment, the mixer comprises a dichroic beam splitter and a nonlinear crystal which might comprise a periodically poled lithium niobate (PPLN) crystal. Optionally, the PPLN crystal is doped with one of MgO and ZnO to raise the damage threshold of the PPLN. The nonlinear crystal may comprise material chosen from LiNbO3, LiTaO3, KTP, RTP, RTA, GaAs, AlGaAs, ZnS, ZnTe, and ZnSeTe, and the probe pulse signal source may comprise a passive mode-locked fiber ring laser.
An embodiment of the invention includes a third harmonic filter and an analog-to-digital converter for receiving said near third harmonic signal after filtration, and optionally a microprocessor for receiving and analyzing output from said analog-to-digital converter.